Next-Event Estimation or “NEE” is a widely used conventional approach to performing ray tracing in the absence of refraction. NEE estimates direct illumination by sampling a light source and testing its visibility by casting a shadow ray. NEE may be performed at each step of light path construction, i.e., at every vertex of a light path, for example.
However, despite its usefulness for estimating direct illumination, NEE is unable to provide accurate estimates for illumination resulting from subsurface scattering of light within a volume enclosed by a refractive boundary. Examples of such volumes include marble statuary, human tissue, milk, and other translucent media. Because NEE connects the light path vertices within a volume enclosed by a refractive boundary with a light source external to the enclosed volume using straight lines through the refractive boundary, the shadow rays utilized by NEE violate Snell's law when crossing the refractive boundary. Thus, a solution enabling ray tracing across refractive boundaries is desirable in order to enhance the accuracy with which illumination caused by subsurface light scattering in translucent media is rendered.